I've had this issue before after swapping the boot order of my HDDs on an older system. XP was installed on an IDE disk and I had linux installed on a second-hand SCSI drive.

If you've recently installed any new hardware or recently installed a second OS, or made changes to your partitions, try to undo those changes.

Otherwise, try tapping F8 during boot until you get the safe mode menu, select "Last known good configuration", and hit enter.

If that doesn't work, try the Memtest86+ reccomended by Wil, and if it finds no problems, boot the XP install disc and re-install. You don't need to reformat or anything, just do a repair or in-place upgrade install. If it won't install because of a service pack you have installed, you'll need to slipstream that onto the XP disc using nLite.